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Acute changes in leucine and phenylalanine kinetics produced by parenteral nutrition in premature infants
S E Clark1, C A Karn, J A Ahlrichs
1Department of Pediatrics, Indiana University School of Medicine, Indianapolis 46202-5210, USA.
Insights
Parenteral nutrition positively impacts leucine and phenylalanine balance in premature infants. However, it increases leucine oxidation while protein synthesis rises and proteolysis decreases.
Area of Science:
- Biochemistry
- Neonatal Nutrition
- Metabolic Research
Background:
- Premature infants often require parenteral nutrition (PN) for growth and development.
- Understanding nutrient metabolism, specifically amino acid kinetics, is crucial for optimizing PN regimens.
- Leucine and phenylalanine serve as key indicators for protein synthesis, breakdown, and catabolism.
Purpose of the Study:
- To investigate the impact of parenteral nutrition on leucine and phenylalanine metabolism in premature infants.
- To assess changes in protein synthesis and proteolysis during intravenous feeding.
- To evaluate the balance and catabolism of specific amino acids under PN.
Main Methods:
- Utilized stable isotope tracer infusions to measure leucine and phenylalanine kinetics.
- Infants were studied under basal conditions and during intravenous infusion of glucose, lipids, and amino acids.
- Analyzed changes in amino acid balance, oxidation, hydroxylation, protein synthesis, and proteolysis.
Main Results:
- Parenteral nutrition shifted both leucine and phenylalanine balance from negative to positive.
- Leucine oxidation significantly increased (2-fold), while phenylalanine hydroxylation remained unchanged.
- Protein synthesis utilization for both amino acids increased, and proteolysis (endogenous release) significantly decreased.
- A potential limitation in phenylalanine/tyrosine supply was suggested by differential catabolism rates.
Conclusions:
- Parenteral nutrition enhances protein synthesis and reduces protein breakdown in premature infants.
- The differing effects on leucine and phenylalanine catabolism highlight potential nutrient supply considerations.
- These findings inform the optimization of parenteral nutrition strategies for preterm neonates.
Abstract:
To determine the effect of parenteral nutrition on the balance and catabolism of leucine (by oxidation) and phenylalanine (by hydroxylation) and to assess any acute changes in proteolysis and/or protein synthesis, leucine and phenylalanine kinetics were measured by stable isotope tracer infusions in nine 32-wk gestation premature infants under both basal conditions and in response to an i.v. infusion of glucose, lipid, and amino acids. Leucine and phenylalanine balance both changed from negative to positive during parenteral nutrition. However, leucine and phenylalanine catabolism were differently affected by parenteral nutrition; the rate of leucine oxidation increased 2-fold, whereas the rate of phenylalanine hydroxylation was unchanged from basal values. Phenylalanine utilization for protein synthesis and leucine utilization for protein synthesis (based on both plasma leucine and alpha-ketoisocaproic acid enrichments) increased significantly during parenteral nutrition. The endogenous rates of release of leucine (based on plasma leucine enrichment) and phenylalanine (both reflecting proteolysis) were significantly reduced during parenteral nutrition. The endogenous rate of release of leucine (based on alpha-ketoisocaproic acid enrichment) was slightly but not significantly lower during parenteral nutrition. The substantial increase in leucine oxidation without changes in phenylalanine hydroxylation suggests a possible limitation in the phenylalanine/tyrosine supply during parenteral nutrition. In addition, these results suggest that premature infants respond to parenteral nutrition with acute increases in whole body protein synthesis as well as a probable reduction in proteolysis.